move moment to peerDependency in elastic-datemath

[nreese]

Summary

EUI is using elastic-datemath in the EuiSuperDatePicker component, elastic/eui#1351. There is a version conflict between elastic-datemath's moment dependency and EUI's moment dependency. Moving the moment dependency to a peer dependency should help alleviate some of these problems.

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[spalger]

code LGTM

[elasticmachine]

Pinging @elastic/kibana-platform

[spalger]

6.x/6.6: c861627